Podocyte depletion is a major determinant of chronic kidney disease. Because podocytes lack the ability to regenerate, the only way that podocytes can cope with ongoing functional stress is by hypertrophy of remaining podocytes. Currently, little is known about the molecular mechanisms that coordinate this process in acquired kidney diseases. In the current proposal, we identify and characterize a novel podocyte to podocyte hypertrophic signal that is activated by injury and coordinates the podocyte injury response. We recently reported that increased expression of the Rap1 activator inhibitor Rap1GAP rendered genetically resistant podocytes sensitive to injury (J Clin Invest, 2014). Examination of injured podocytes in murine kidney disease models and kidney biopsies from glomerulosclerosis patients revealed that podocyte injury was associated with enhanced Rap1GAP expression and subsequent Rap1 inhibition. Here, we identify synaptogagmin-like protein 2a (Slp2a), an exocytosis effector molecule, as a direct binding partner of Rap1GAP. Similar to Rap1GAP, expression of Slp2a is massively increased in injured podocytes both in mouse models and in human glomerulosclerosis patients. Our preliminary data in cultured podocytes suggest that over-expression of Slp2a results in a paracrine signal that drives cellular hypertrophy of adjacent cells by a mechanism that requires mTORC1 activation. Furthermore, genetic deletion of Slp2a in mice abrogates compensatory podocyte hypertrophy after uni-nephrectomy resulting in proteinuria and accelerated glomerulomegaly. We hypothesize that after podocyte injury, increased formation of a protein complex that includes Rap1GAP and Slp2a drives compensatory hypertrophy by stimulating release of podocyte-derived proteins that then activate mTORC1 in adjacent cells. The goals of this grant are to definitively prove the existence of such podocyte crosstalk. In aim 1, we examine the mechanisms of Slp2a-mediated cellular hypertrophy that include proteomic studies to identify the secreted hypertrophic factor. In aim 2, we characterize podocyte hypertrophy in a novel mouse model that allows for stochastic Slp2a expression in individual podocytes. In this way, we will be able to determine the ability of enhanced Slp2a expression to induce hypertrophy both in an individual podocyte and in its neighboring cells. Finally, in aim 3, we build on our preliminary finding that absence of Slp2a in mice abrogates compensatory podocyte hypertrophy. At the conclusion of these studies, we will have defined a novel type of podocyte crosstalk as a mediator of compensatory hypertrophy, the only available mechanism by which podocytes try to adapt to ongoing stressors induced by podocyte loss. This will represent a major step in our understanding of the mechanisms of proteinuria.